The design of duplex dual conversion transceivers requires the inclusion of multiple frequency synthesizers to receive and transmit simultaneously in the same or in different frequencies for either Time Division Duplex (TDD) or Frequency Division Duplex (FDD) operation, respectively. In a FDD system the modulated intermediate frequency (IF) signal is generated with the combination of the offset local oscillator (LO) synthesizer and baseband signals. In a TDD system the second local oscillator (LO) signal is combined with the baseband signal to generate the modulated intermediate frequency which, in turn, is mixed with the main LO to produce the carder frequency.
A block diagram of a typical FDD radio is shown in FIG. 1 of the accompanying drawings. The FDD system uses three separate synthesizers a main LO 114, a second LO 116, and an offset LO 110. The offset LO synthesizer 110 supplies the intermediate frequency for the TX, transmit, path. The second LO 116 is used for the RX, receive, path. In a typical FDD system 100, an incoming audio signal 104 received at the microphone 102 is converted to an encoded audio signal, which will be referred to as TX baseband signal 108, through a controller section 106. The TX baseband signal 108 is then combined with an offset LO signal 111, generated by the offset LO synthesizer 110, at the modulator 112 to generate a modulated IF signal 124. The modulated IF signal 124 is then mixed with a main LO signal 122, generated by the main LO synthesizer 114, at the TX mixer 126. This produces a TX carrier frequency 132 that in turn is filtered at filter 134 and amplified at amplifier 138, generating an amplified RF (radio frequency) signal 140. The amplified RF signal 140 is then coupled through an antenna coupler, such as duplexer 142, to an antenna 146 where it is transmitted as RF signal 144.
In the receive, RX, path for the FDD system 100, the RF signal 144 is received at antenna 146 and coupled through the duplexer 142, for directing the RF signal to the RX path, as RX signal 148. The RX signal 148 is then amplified through amplifier 150 and filtered through filter 154. A filtered signal 130 is then mixed with the main LO signal 122 at RX mixer 128 to produce an IF carrier signal 120. The IF carrier signal 120 is fed into a zero IF section (ZIF) 118 where the IF signal is converted directly to a RX baseband frequency 156 by mixing the incoming IF carrier signal with a second LO signal 121, generated from the second LO synthesizer 116, at the same frequency as the IF carrier signal. The baseband signal 156 is then processed for timing and digital to analog conversion in the controller section 106 and output as audio at speaker 158.
In a typical TDD radio shown in FIG. 2 an audio signal 204 is received at microphone 202 and processed in a controller section 206 where the audio is converted to a an encoded audio signal which will be referred to as TX baseband signal 208. A second LO synthesizer 252 generates a second LO signal 212 which gets combined with the TX baseband signal 208 at modulator 210 to generate a modulated IF signal 214. The modulated IF signal 214 is then mixed at TX mixer 218 with a main LO signal 216, generated from a main LO synthesizer 217, to produce a TX carrier frequency signal 220. The TX carrier 220 is then amplified at amplifier 222 to generate an amplified TX carrier signal 224 and switched to the TX path at antenna switch 226 then filtered at filter 228 and transmitted at an antenna 240 as transmit RF signal 230.
In the receive mode of the TDD system, the RF signal 230 is received at antenna 240, filtered at filter 228, and switched through antenna switch 226 to the RX path as RX signal 242. The RX signal 242 is then amplified through amplifier 244 and mixed with a main LO signal 216, generated from a main LO synthesizer 217, at a RX mixer 248 to generate a RX IF carrier signal 250. The RX IF carrier signal 250 is combined with the second LO synthesizer signal 212 at a ZIF 254 to form a RX baseband signal 256. The RX baseband signal 256 is forwarded to the controller section 206 where it is processed and converted to audio at speaker 258.
The FDD and TDD systems as described by FIGS. 1 and 2 show the systems as two separate radios. A device that would provide a combination of platforms in one embodiment would be useful to system and radio designers trying to design cost effective radios while reducing design time.